Water exists just about everywhere on Earth.
Water exists just about everywhere on Earth. Of the Earth's 331 million cubic miles of total water, 96.5 percent is found in the oceans. Of the remaining freshwater, most is locked up in glaciers and icecaps or found in groundwater and surface water. One theory suggests that much of Earth's water actually came from comets and asteroids hitting the planet over billions of years.
Oceans
Seen from space the Earth appears blue because of the oceans, which dominate 70.8 percent or 139 million square miles of the surface. The average depth of these seas is 2.4 miles and maximum depths can reach 6.2 miles in several ocean trenches. While the oceans contain 96.5 percent of the Earth's water, it is unevenly distributed. The ratio of land to ocean in the Northern Hemisphere is one to one and a half while in the Southern Hemisphere it's one to four. Because it has more ocean surface, the climate of Southern Hemisphere is often more moderate than the Northern Hemisphere.
Fresh Water
Glaciers contain much of Earth's fresh water.
Only about 3 percent of all Earth's water is freshwater. The majority of this -- about 68 percent - is stored in polar icecaps, glaciers, and permanent snow, mainly in Greenland and Antarctica. Another 30 percent of the Earth's freshwater is actually under the ground in saturated geologic formations, much of it unattainable. The remaining 0.3 percent is contained in rivers and lakes - about 22,300 cubic miles - where most of the water we use everyday exists.
The Water Cycle
The Earth's water exists in a "closed system" much like a terrarium. This means the planet neither gains nor loses the available water, and the same water that existed millions of years ago still exists. This is due to a water cycle that recycles the water all around the globe. This water may begin as ice from a glacier that melts and flows into a river. That river water may flow into a lake or the ocean. Evaporation then turns that water into vapor that rises into the atmosphere where it forms clouds. Finally, those clouds turn into storms and drop the water as either rain or snow and the cycle continues.
Ancient Origins
Comets have have supplied Earth's water.
Scientists estimate that large amounts of water have flowed on Earth for the past 3.8 billion years, and water is believed to have initially surfaced through the eruptions of ancient volcanoes. A 2006 study by the University of Hawaii provided evidence that icy comets may have served as a major source for Earth's water as they crashed into the Earth over millions of years. A second study, reported in the April 28, 2010 issue of Discovery News, found the first evidence of ice on asteroids. With that study, scientists are now suggesting that both comets and asteroids crashing into the Earth four billion years ago supplied the planet's first water.
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